1. Field of the Invention
This invention relates to laser imaging systems and, in particular, to a laser imaging system permitting viewing under simultaneous laser and white light illumination for use in analyzing defects on semiconductor wafers.
2. Related Art
Semiconductor chip manufacturers have increasingly sought to improve yields in their production processes. Key to this effort is the reduction of particulate contamination during wafer processing. As the line widths of features on the chip have shrunk from 1.0 microns several years ago to 0.35 micron and below today (with line widths approaching 0.18 micron or less expected in the next few years), the ability to detect and control smaller and smaller particles to achieve higher degrees of cleanliness has become paramount. Additionally, production of acceptable chips requires accurate performance of each of the process steps carried out on the wafer. The value of product on each wafer has also increased dramatically, due to the increasing complexity of semiconductor devices (many more layers and process steps) and the development of larger wafers (up to 300 mm diameter), further accentuating the need for defect detection and control.
Instrument suppliers have addressed a portion of this problem by developing defect detecting systems which scan wafers (wafer scanners) during production for anomalous optical sites that are characteristic of particulate contamination (but may represent other flaws as well). An review of currently commercially available defect analysis devices is presented in commonly owned U.S. Pat. No. 5,479,252, entitled "Laser Imaging System For Inspection and Analysis of Sub-Micron Particles," by Worster et al., issued Dec. 26, 1995, the disclosure of which has incorporated by reference.
Laser imaging systems for sub-micron particle structure evaluation provide advantages over other types of defect analysis devices including allowing hands-off operation under class 1 cleanroom conditions, and "revisiting" defects on production semiconductor wafers, where the defects are first detected (but not analyzed or evaluated) by conventional wafer scanners.
As noted above, the decreasing line widths of features on current and future semiconductor chips increase the importance of detection of contaminants and other defects having a diameter, width, or other characteristic dimension on the order of 0.1 to 0.3 microns. The visible light, off-the-shelf microscopes currently being used in defect review stations lack sufficient resolution to resolve defects of such small size, or to resolve this size structure on larger defects to aid in identification. Visible light scanning microscopes (both white light and laser-based) that are built by modifying off-the-shelf microscopes can improve the resolution significantly, but they are currently in limited use, mostly as part of complex and expensive research setups. Additionally, the use of conventional microscopes increases the risk of contamination of the semiconductor chips during the review process, since a (relatively dirty) human is in close proximity to the wafer surface and because the presence of the microscope causes turbulent flow near the wafer which tends to pull in nearby contaminants to the wafer.
Unlike other devices used for semiconductor defect analysis, a laser imaging system will not damage samples or slow processing, and costs significantly less to implement than, for example, a scanning electron microscope (SEM). Moreover, while SEMs can produce images with resolution on the nanometer scale, they have certain limitations. For example, the SEM image has an extended depth of field, like a photograph taken through a high f-stop aperture, but this image contains no quantitative depth information. Some methods of dealing with this deficiency are sample tilting or coating to produce a "shadowing" effect or perspective change, but these methods require additional process steps and cost, may damage the wafer, and do not completely resolve the problem.
Laser imaging systems are capable of operating in cleanrooms having a class 1 cleanroom requirement. Laser imaging system are capable of producing a three dimensional image, using simple image rendering techniques, which provides quantitative dimensional information. The image can be stored and recalled for later viewing. The image can be rotated or tilted or shaded, with correct perspective maintained, without necessity for sample tilting or coating. Additionally, the laser imaging system has the ability to perform sub-surface viewing of defects lying beneath dielectric layers, an ability unavailable to the SEM. Combined with three-dimensional analysis software, a user is able to examine cross sections of the defect and surrounding material, and to assess the impact on circuit layers of the wafer. Since laser imaging systems are capable of resolutions on the order of 0.1 to 0.2 microns, they may be used for metrology as well.
A laser imaging system typically utilizes confocal laser scanning microscopy techniques, including multi-line visible light lasers, and can be optionally fitted with an ultraviolet laser, improving resolution even further due to the shorter wavelengths of the ultraviolet light. White light imaging, in addition to laser imaging is featured in laser imaging system. White light imaging permits conventional microscope images to be attained, and by use of a video camera or charge coupled device (CCD), the image may be displayed on a video monitor or computer display without undesirable proximity of the operator to the wafer.
The white light microscope image may be produced either alone or simultaneously with the live laser image. Simultaneous viewing under both white light and laser light is desirable in that the white light image provides color information of the defect and facilitates locating and identifying the defect, while the laser confocal image provides three-dimensional information of the defect.
To enable simultaneous viewing under both white light and laser light, a color filter is positioned between the illuminated portion of the wafer and the CCD camera to block the laser light from entering the camera. For example, where an argon ion laser is used to illuminated the wafer, a color filter blocking any wavelengths below 550 nm will effectively block off the laser lines between 458-515 nm. The purpose of the filter is to prevent the reflected laser light from over-saturating the image at the video camera, thus permitting the camera to acquire the white light image without being "blinded" by the laser.
The approach of currently available laser imaging systems, however, suffer serious limitations in their ability to provide simultaneous viewing of a semiconductor wafer under both white light and laser light. For example, the color filter typically used to filter out the laser light from the CCD camera removes a significant portion of the white light spectrum resulting in an unnaturally hued white light image. For example, where a 550 nm cut-off filter is used to remove wavelengths associated with an argon ion laser, the white light picture is rendered yellow, thus obscuring some colored defects. Moreover, in laser imaging systems having simultaneous viewing capability, the autofocus feature is typically performed with the color filter in place. Accordingly, if a user wishes to visit defects in white light only, the color filter must be switched in and then out of the optical path, thus significantly increasing the process time required to investigate a defect.
A further limitation of laser imaging systems having simultaneous white light/laser light viewing capability are the laser light interference fringes caused by the bright field cube of the microscope optics. The effect of these interference fringes is to produce bands of increased contrast in the laser image. These bands tend to obscure details of the observed structures on the semiconductor wafer, thus compromising the resolution of the confocal laser image.
Accordingly, there is a need for a laser imaging system capable of simultaneously producing white light and laser confocal images without compromising the color integrity of the white light image, and without producing a laser confocal image having contrast bands associated with interference fringes produced in an optical element of the laser imaging system. There is also a need for a laser imaging system capable of simultaneous imaging under both white light and laser light that permits a white light imaging mode only without having to remove and then re-insert the color filter during autofocus.